Thanks to a malformed chemical structure, the deformed ear bones end up being bigger, lighter and more brittle than they should be. This impacts proper hearing in the animal.

The prevalence of the deformity – 10 times more likely in farmed fish than wild, regardless of species - was uncovered by researchers from the University of Melbourne, who chose to study Atlantic salmon farms from the world's top salmon producers: Canada, Chile, Norway, Scotland and Australia.

"The deformity occurs at an early age, most often when fish are in a hatchery, but its effects on hearing become increasingly more severe as the fish age," explained the study's lead author Tormey Reimer in a statement, adding that the deformity can cost a fish up to 50 percent of its hearing.

The scientists compared ear structures in both farmed and wild salmon from the top-producing countries and also used a mathematical model to predict what the fish would be able to hear based upon their ear structures.

The researchers found that no matter which country's fish they studied the ear bone deformity was much higher in the farmed fish versus wild.

"We estimate that roughly half of these fish have the earbone deformity and thus have compromised hearing," said Reimer. "We don't yet know exactly how this hearing loss affects their performance in farms."

"However," she added, "producing farmed animals with deformities contravenes two of the 'Five Freedoms' that form the basis of legislation to ensure the welfare of farmed animals in many countries."

"We now need to work out what is the root cause, to help the global salmon industry produce fish with acceptable welfare standards," Reimer said.

It's a fish-eat-fish world, so many fish have evolved unusual body shapes to deter other fish from swallowing them, new research finds.
The study, published in the journal Proceedings of the Royal Society B, points out that biting off more than one can chew is not an option for most aquatic predators. As a result, thousands of prey species have evolved bodies that are ultra wide, round, narrow, spiked or are otherwise designed not to be the perfect morsel.
"Almost every fish that eats other fishes captures their prey with their mouths, and so the prey must fit into their mouth; this is also true for some fish-eating birds, such grebes and herons," lead author Samantha Price of the University of California at Davis' Department of Evolution and Ecology told Discovery News.
"Not all predators are gape-limited, but over the history of fishes, it is likely that many of the predators would have been limited by the size of their mouth."
This sea devil can be both predator and prey, as its human-like mouth, yet flat body shape, suggest. There are multiple reasons for "misshapen" bodies among fish, but avoiding fitting into a hunter's mouth turns out to be a critical one.

Imagine trying to fit this into your mouth. The flying gurnard makes maximum use of body area without investing much energy in fleshy substance.
The new study, which looked at over 25,000 living fish species, found that evolution of fish such as this was shaped by gape-limited predators, meaning marine hunters that can only open their mouths so far.

In addition to body shape, many fish have evolved bumpy skin, fin spines and other features that deter a predator from simply sucking the fish into their mouth like a smooth piece of sushi. The researchers determined that fish with spines that extend vertically evolve toward "deeper" bodies. Deep in this case means having a more vertical yet narrow, rounded body, sort of like a pancake on its side.
The slantbrow batfish is just the opposite. It is considered to be a wider-bodied fish, like a flat, albeit misshapen, pancake floating in the water. These wider-bodied fish often evolve spines that extend horizontally.
Price said that the spines and body depth "work together to defend against predators."
She continued that if the prey fish "becomes bigger, the predator will not be able to eat it, but increasing the overall size is energetically costly, so it is better just to increase a single dimension: either width or depth."

On the predator side are fish like this giant grouper. Sharks often grab our attention because of their appearance and apex predator status, but groupers like this have been known to eat sharks from time to time using their huge mouths and relentless hunting tactics.

The leafy sea dragon belongs to the marine fish family that also includes seahorses and pipefish. Its small fins not only increase its body surface area, but they are also difficult to view as they subtly undulate while the seadragon moves slowly through the water.
Here, camouflage is another benefit of body shape. When in motion, this seadragon looks remarkably like a piece of floating seaweed.

"Fishes display an incredible diversity of body sizes, including lineages that are almost as wide or as deep as they are long," Price said.
"Our findings suggest that predation has helped shape this diversity, although many other factors, such as habitat or diet, are also likely to be important drivers of fish body form evolution."

This red lionfish looks to be posing for the camera, displaying its showy colors and fins. Predators would do well to notice and move away, as lionfish are venomous.
Spines on such fish therefore are doubly dangerous.
"The spines can damage the predator and species that have venom associated with the spines will cause even more problems," Price explained. "There are some rather grisly photos online of some injuries to fishermen and swimmers who have been on the wrong end of fish spines!"

Moonfish are classic examples of fish that have evolved extremely deep body sizes that the researchers say deter gape-limited predators. The moonfish body is also sharp-edged and covered on the top and bottom with small spines that make handling them challenging.
Brian Sidlauskas is an associate professor and curator of fishes at Oregon State University's Department of Fisheries and Wildlife. He told Discovery News that the new study provides "an elegant demonstration" of at least one "critical piece" of fish body form evolution.

When deflated, puffer fish look like helpless, tiny prey. Once inflated, though, they become deadly mouthfuls for many would-be diners.
Prosanta Chakrabarty of Louisiana State University agrees that spines not only work against predators via their sharpness and possible venom, but also because they help certain fish to become wider or deeper bodied.
"That might sound intuitive, but it really isn't," he informed Discovery News. "Your bodies are constrained in how deep or wide they can get because these animals still need to try to avoid predators, get their own prey, move around the water column, and find mates."
He continued, "The spines, which are not similarly constrained, allow these fishes to reach these depths and widths despite the constraints on the body. That is a microevolutionary solution to a macroevolutionary constraint."